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RE: Livezey and Zusi's big bird morph analysis [...]

With regard to the putative morphological convergence of foot-propelled
divers (and other dinosaurian examples mentioned), I'm reminded of a far
more pervasive convergence within snakes, for burrowing. 
You can remove any convergence between two groups (and hence error it may
cause) in an analysis by deleting one of them; i.e. if there are n putative
convergers, run the analysis n times including one at a time along with
'generalised' taxa. I did this with snake morphological data in an
unpublished bit of my PhD, and essentially the same method was later
proposed by Siddall and Whiting (1999) to beat LBA in sequence data.
What happens when you delete grebes/loons in separate analyses of L&Z (or
any prior avian morphology dataset)? Do they fit in the same place relative
to other taxa? - if so, there's no reason to assume convergence.  
I'm sure the statistics of this approach would have been investigated more
thoroughly since SW99 in the sequence context, though I haven't followed it
up. Given the usual arguments for including all available taxa, this
reciprocal exclusion approach seems a pretty rough method and can't result
in very high confidence, but I'd be interested to see something better.
Dr John D. Scanlon
Riversleigh Fossil Centre, Outback at Isa
19 Marian Street / PO Box 1094
Mount Isa  QLD  4825
Ph:   07 4749 1555
Fax: 07 4743 6296
Email: riversleigh@outbackatisa.com.au

> -----Original Message-----
> From: Tim Williams [mailto:twilliams_alpha@hotmail.com]
> Sent: Saturday, February 03, 2007 1:34 PM
> To: dinosaur@usc.edu
> Subject: Re: Livezey and Zusi's big bird morph analysis [...]
> Evelyn Sobielski wrote:
> >But the case for convergence confounding analyses has
> >been made, time and again, for exactly this clade. It
> >is THE showcase example where parsimony is likely to
> >run into a wall*, and has been since more than a
> >decade (in fact, try doi:10.1007/BF01908745). This
> >cannot be dismissed out of hand. Basically they
> >reiterate a cold case; this is not good.
> But why do you assume that the morphological characters shared by grebes
> and
> loons *are* due to convergence?  I agree with David here.  When it comes
> to
> assessing relationships between grebes and loons, why is the
> convergence/homoplasy hypothesis better supported than the common ancestry
> hypothesis?  Why can't the characters associated with foot-propelled
> diving
> also be potential synapomorphies?
> This issue also comes up with plotopterids and penguins w.r.t. subaquatic
> diving characters; with birds of prey (traditional falconiforms and
> strigiforms) w.r.t. to predatory characters; with theropods and
> herrerasaurs
> too w.r.t. predatory characters; with euhelopodids and mamenchisaurs
> w.r.t.
> neck elongation; with dinosaurs and pterosaurs w.r.t. bipedalism...  And
> so
> on.
> Cheers
> Tim
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